A group-III nitride semiconductor is a group III-V semiconductor in which nitrogen is used as a group-V element. Representative examples of the nitride semiconductor include aluminum nitride (AlN), gallium nitride (GaN) and indium nitride (InN). In general, the nitride semiconductor can be represented as AlxInyGa1-x-yN (0≦X≦1, 0≦Y≦1, 0≦X+Y≦1).
A HEMT (high-electron-mobility transistor) using such a nitride semiconductor is proposed. A HEMT includes, for example, an electron transit layer that is formed of GaN and an electron supply layer that is epitaxially grown on the electron transit layer and that is formed of AlGaN. A pair of a source electrode and a drain electrode are formed so as to be in contact with the electron supply layer, and a gate electrode is arranged therebetween. The gate electrode is joined to the electron supply layer with a Schottky junction or is arranged so as to be opposite to the electron supply layer across an insulating film. Due to polarization caused by the lattice mismatch between GaN and AlGaN, within the electron transit layer, in a position a few angstroms inward from an interface between the electron transit layer and the electron supply layer, a two-dimensional electron gas is formed. The two-dimensional electron gas is used as a channel, and thus a connection is made between the source and the drain. When a control voltage is applied to the gate electrode to interrupt the two-dimensional electron gas, an interruption occurs between the source and the drain. Since in a state where the control voltage is not applied to the gate electrode, electrical continuity is established between the source and the drain, the device functions as a normally on-type device.
Since the device using the nitride semiconductor has features such as a high-voltage resistance, a high-temperature operation, a high-current density, high-speed switching and a low on-resistance, the application to power devices is being examined.
However, since the device needs to be a normally off-type device which interrupts a current at the time of zero bias for an application for a power device, the HEMT described above cannot be applied to the power device.
A structure for realizing a normally off-type nitride semiconductor HEMT is proposed in, for example, Patent Document 1.
On the other hand, Patent Document 2 discloses a GaN-based semiconductor device that is produced as follows: a plurality of GaN-based HEMTs (high-electron-mobility transistors) are formed on a silicon substrate and the electrodes of the GaN-based HEMTs are coupled to each other by multilayer wiring. On the silicon substrate, a buffer layer and a semiconductor operation layer are formed. The semiconductor operation layer is separated into a plurality of semiconductor operation layer regions by an insulating region formed by ion implantation.